In the production of rigid foam billets, boards, slabs, logs, or the like, a foamable thermoplastic resin, such as styrene or the like, incorporating a blowing agent is extruded through a die orifice. As soon as the extrudate leaves the die orifice, it is at a much lower pressure, preferably a vacuum, and immediately begins to expand three dimensionally and move away from the die orifice. If such expansion is not properly controlled and the extrudate properly drawn from the die orifice, particularly as the extrudate cools and begins to cure or to set, undesirable dimensional variations may result, such as corrugations and density and/or cell orientation variations. Also, it is desirable to finish the product as nearly as possible at a uniform thickness with planar top and bottom surfaces as the product is so formed. The finished product, then, need only be cut transversely with perhaps lateral edge trimming, or tongue and grooving. It is important that the product be of uniform density and uniform thickness and have minimum cell distortion; and it also is important that the finished surface be free of distortions, surface blemishes or cracks.
In U.S. Pat. No. 4,234,529 there is disclosed a method and apparatus for shaping plastic foams extruded through an arcuate extrusion die with the extrudate being conducted past several sets of curved rolls. Each set of curved rolls includes two of the same respectively located on opposite sides of the extrudate pass line. The center of curvature of each curved roll is approximately at the center of curvature of the arcuate extrusion die orifice. At least one curved roll of each set thereof is independently driven to rotate the same, and at least one curved roll of each set thereof is movable to adjust the spacing between the same and the other curved roll of such set. The extrusion die, curved rolls, and a surface finisher, if employed, are located in a controlled environment, such as a vacuum.
While angularly related high pressure pinch rolls are employed for stretching of non-foaming extrudates such as in biaxially orienting film, as seen in Japanese publication No. 11598/71, an expanding board or billet of some thickness presents problems of a different sort, particularly where the foaming and shaping occurs in a vacuum.
The several features and advantages of the method and apparatus disclosed in U.S. Pat. No. 4,234,529 also are relevant to the instant invention. Accordingly, the entire disclosure of such patent hereby is incorporated by reference.
It is desirable to provide additional forming and shaping capability for an extruding apparatus, such as that disclosed in such patent, especially to facilitate maintaining high quality in the production of relatively wide boards or billets of extruded plastic foams.